Some wireless communication systems utilize a series of base stations to provide radiocommunications to mobile subscriber units located in a given territory or geographic region. One constraint imposed on such wireless communication systems is a limitation on the amount of power that the base stations and the mobile units can output when communicating with each other. This limitation results in each base station and each mobile unit having a limited range of communication.
To extend the range of communication or coverage between a mobile unit and a base station, some wireless communications systems use repeaters. In general, the repeaters are composed of an input antenna for receiving a signal, an amplifier circuit for amplifying the received signal, and an output antenna for outputting the amplified signal. Thus, the range of communication or coverage of a base station can be increased by placing a repeater in a location such that a signal sent from the base station is received by the input antenna, amplified by the amplifier circuit, and output through the output antenna. In this capacity, the repeater increases the strength of the base station signal so that the base station signal can be received by a mobile unit at greater distances from the base station.
Since it is common for repeaters having the above structure to transmit the amplified signal from the output antenna at the same frequency as the signal received at the repeater input antenna, such repeaters are usefull in wireless communication systems only when there is sufficient isolation between the input antenna and the output antenna of the repeater. A repeater is said to have sufficient isolation when the signal transmitted from its output antenna (i.e. amplified signal) does not interfere with or prevent the accurate reception of the input signal received through its input antenna. For example, sufficient isolation is not found where the signal transmitted from the output antenna is received by the input antenna, thus causing the amplifier circuit to oscillate.
One method for providing sufficient isolation between the input antenna and the output antenna of a repeater is to output the amplified signal through the repeater output antenna at a different frequency than the signal received through the repeater input antenna. This method, however, suffers from the major drawback that it prevents a mobile unit from being able to directly communicate with both the base station and the repeater. This is due to the fact that the mobile unit, which has the ability to be tuned to only one frequency, can not be tuned to both the frequency transmitted by the base station and the frequency transmitted by the repeater at the same time. As a result, if the mobile unit is tuned to the frequency of the signal transmitted from the output antenna of the repeater, the mobile unit will not be able to communicate directly with the base station, and vice verse. Thus, such a method for providing sufficient repeater isolation is not desirable for wireless systems wherein it is desirable for the mobile units to communicate with both base stations and repeaters.
Another method for providing sufficient isolation between the input antenna and the output antenna of a repeater is disclosed by Marque-Pucheu in U.S. Pat. No. 5,509,028, issued Apr. 16, 1996, and incorporated herein by reference. Marque-Pucheu discloses a repeater that uses a technique called spectrum reversal to provide isolation between its input and output antennas. A repeater employing spectrum reversal basically reverses the spectrum of a signal received through the repeater input antenna, and outputs the reversed-spectrum signal through the output antenna. This method, however, suffers from the same drawback as the method involving the use of different frequencies, as described above. That is, a mobile unit can not be tuned to receive both the reverse-spectrum signal from the repeater and the non-reverse-spectrum signal from a base station. Thus, such a method for providing sufficient repeater isolation is not desirable for wireless systems wherein it is desirable for the mobile units to communicate with both base stations and repeaters.
Another method for providing sufficient isolation between the input antenna and the output antenna of a repeater is disclosed by Justice et al. in U.S. Pat. No. 5,600,333, issued Feb. 4, 1997, and incorporated herein by reference. Justice et al. disclose the use of electromagnetic shielding between the input antenna and the output antenna. The electromagnetic shielding essentially prevents the signal transmitted from the repeater output antenna from being received by the repeater input antenna. As a result, the shielding can prevent repeater oscillation, and thus provide sufficient repeater isolationBy
By placing an electromagnetic shield between the input antenna and the output antenna of the repeater, however, the communication range of the repeater may be significantly reduced. That is, the electromagnetic shield may block the amplified signal output from the repeater output antenna such that the amplified signal propagates in only one direction that is perpendicular to the electromagnetic shield. As a result, a mobile unit located in different direction from the output antenna may not receive the amplified signal from the repeater, thus limiting the ability of the repeater to extend the range of communications of the base station.